Interleukin 8 (IL-8) is mitogenic and chemotactic for endothelial cells. Within a neoplasm, IL-8 is secreted by inflammatory and neoplastic cells. The highly tumorigenic and highly metastatic human transitional cell carcinoma (TCC) cell line 253J B-V overexpresses IL-8 relative to the nontumorigenic and nometastatic 253J-P cell line. To determine whether IL-8 expression regulates tumorigenicity and metastasis in human TCC, 253J B-V cells were transfected with the full-sequence antisense (AS) cDNA for IL-8, whereas 253J-P cells were transfected with the full-length IL-8 cDNA, and control cells for each were transfected with the neomycin resistance (Neo) gene. In vitro, sense-transfected 253J-P cells overexpressed IL-8-specific mRNA and protein, whereas both of these were markedly reduced in AS-IL-8-transfected 253J B-V cells relative to controls. Moreover, sense-transfected cells showed up-regulation in matrix metalloproteinase type 9 mRNA, collagenase activity, and increased invasiveness through Matrigel-coated filters, whereas these measures were lower in AS-transfected cells relative to controls. After implantation into the bladders of athymic nude mice, the sense-transfected 253J-P cells acquired increased tumorigenicity and metastasis, whereas the AS-transfected cells significantly inhibited tumorigenicity and metastases in the 253J B-V cell lines. This effect was accompanied by reduced IL-8 expression and microvessel density. These studies demonstrate that IL-8 expression enhances angiogenic activity through the induction of matrix metalloproteinase type 9 and subsequently regulates the tumorigenesis and production of spontaneous metastases of human TCC.